Dormancy, state of reduced metabolic activity adopted by many organisms under conditions of environmental stress or, often, as in winter, when such stressful conditions are likely to appear.

In plant physiology, dormancy is a period of arrested plant growth. It is a survival strategy exhibited by many plant species, which enables them to survive in climates where part of the year is unsuitable for growth, such as winter or dry seasons.

Plumeria dormancy

Plumeria dormancy is a period of arrested growth. It is a survival strategy exhibited by many plant species, which enables them to survive in climates where part of the year is unsuitable for growth. Plumerias are tropical and a phase of innate dormancy coinciding with an unfavorable season, such as winter or dry seasons. 

Plumeria species that exhibit dormancy have a biological clock that tells them to slow activity and to prepare soft tissues for a period of cooling temperatures or water shortage. This clock works through decreased temperatures, shortened photoperiod, or a reduction in rainfall.

Seed dormancy

Plumeria seeds do not germinate as soon as they are formed and dispersed. They wait until favorable conditions are present. Thus, dormancy helps to keep the seed viable for months or even years. Plumeria seeds have been known to germinate after 10 years or more. However, the germination rate does decrease over the years.

A plumeria seed dormancy is considered to be seed coat dormancy or external dormancy and is caused by the presence of a hard seed covering or seed coat that prevents water and oxygen from reaching and activating the embryo.

Under normal conditions, the plumeria seed matures on the tree in approximately 9 months. The seeds will stay in a dormant state until they are exposed to warm temperatures and moisture. In nature after the seed pod opens the seed coat is weakened by moisture and warm temperatures.

Causes of Dormancy

The dormant state that is induced in an organism during periods of environmental stress may be caused by a number of variables. Those of major importance in contributing to the onset of dormancy include changes in temperature and photoperiod and the availability of nutrients, water, oxygen, and carbon dioxide. In general, because organisms normally exist within a relatively narrow temperature range, temperatures above or below the limits of this range can induce dormancy in certain organisms. Temperature changes also affect such other environmental parameters as the availability of nutrients, water, and oxygen, thus providing further stimuli for dormancy. The lack of water during summer periods of drought or winter periods of freezing, as well as annual changes in the duration and intensity of light, particularly at high latitudes, are other environmental factors that can induce dormant states.

Under natural conditions, most of the environmental variables that influence dormancy are interrelated in a cyclical pattern that is either circadian or annual. Fluctuations in the major daily variables—light and temperature—can induce rhythmical changes in the metabolic activity of an organism; annual fluctuations in temperature and photoperiod can influence the availability of nutrients and water.

Since plumeria can live for many decades or even centuries they must have mechanisms in place which allow them to survive dry periods. Dormancy is a phase in development that allows plumeria to survive these unfavorable conditions. Plumeria are tropical plants and overall cold hardiness will vary even in dormant plants, however, exposure to freezing temperatures will kill a plumeria plant.

Stages of Dormancy

The development of dormancy typically occurs in phases.

• The first phase is termed pre-dormancy. This early phase is reversible in that if the plumeria is returned to favorable growing conditions, in a greenhouse, for example, it will resume growth. As pre-dormancy develops the range of environmental conditions that allow growth to resume narrows.
• Following pre-dormancy if the plumeria enters true-dormancy. In true-dormancy growth will not resume even if the plant is returned to optimal growing conditions. It is believed that plumeria never enters into a true-dormancy state. The plumeria is often defoliated at this point, and a period of prolonged chilling is required before growth resumes.
• The final stage of dormancy is post-dormancy. This stage is typical of later winter and early spring. In post-dormancy, the plumeria is capable of growing, but it is still suppressed by adverse environmental conditions (e.g. low temperatures).

Environmental Triggers

• LENGTH OF DAYLIGHT – The main environmental signal which triggers the onset of dormancy is the length of daylight. For most plumeria, long days promote vegetative growth and short days trigger dormancy. As days begin to get shorter in later summer growth slows, and eventually going dormant. It is actually the length of the night that is critical, not the length of the day. Short nights stimulate growth, long, uninterrupted nights stimulate dormancy. Length of daylight, of course, is a very reliable environmental signal since it is perfectly stable from year to year and plumeria will not be tricked into growing longer because of an abnormally warm fall. Length of daylight then is the primary trigger that results in the changes in growth regulator production which in turn results in dormancy development. The growth regulator abscisic acid (ABA) apparently plays a role in dormancy development and has been found to build up to high levels in the fall.

• INFLUENCE OF TEMPERATURE – Decreasing temperatures also play a role in dormancy development. Short days cause the plumeria to enter pre-dormancy (and maybe even true-dormancy). It is believed by some researchers that cool temperatures are needed for the plant to enter true dormancy. Whatever the specific case, dormancy in many plumerias develops more quickly when short days occur in combination with cool temperatures.

• INFLUENCE OF WATER AND NUTRITION – Both water supply and mineral nutrition also interact with dormancy induction. Water stress deepens dormancy and will result in defoliation. High mineral nutrition can result in delaying dormancy. This is particularly true with mineral nitrogen. High levels of nitrogen should never be given to plants in late summer or early fall since they may actually flush and resume growth. During dormancy do not fertilize and mildly water or mist stress plants only if signs of dehydration are visible.

Release from Dormancy

Some researchers believe that during short days in the fall ABA builds up to high levels and induces dormancy. Chilling may be responsible for the breaking down of ABA. Until enough hours have accumulated to remove the inhibitory effect of ABA the plumeria will not break dormancy. When the soil begins to warm, promoters of growth such as gibberellin and cytokinins build-up, signaling the plumeria tips to resume growth.

Once the plumeria is in a post-dormant condition, warm temperatures and increasing day lengths are required for normal shoot expansion. Warm temperatures are probably the most critical environmental factor at this point.